He is doing a rattlesnake impression, and that is exactly what it is for. The tail buzzing against dry substrate produces a noise close enough to a rattle that a predator which has learned to leave rattlesnakes alone thinks twice. Rat snakes, gophers, bulls and milks all do it.
So it means startled or defensive in that moment, not stressed as a condition and not ill. Eight months old and two months in your house is precisely the profile: young snakes are more defensive and new ones are still settling.